I. SLAB SETTLEMENT:
Slab settlement within a house is fairly common. Since slabs are rarely considered structural members, small cracks in them are usually not to be feared, unless they are performing structural functions. Some houses utilize steel reinforced slabs and footings that are structural members. Cracks in these slabs are almost always warnings of structural problems. These slabs are sometimes called “floating slabs” and are usually found in expansive soil areas or earthquake tremor areas.
The cause of the settlement should be thoroughly understood before any repairs are contemplated. Water can cause soil to consolidate, and it is soil that supports basement slabs (Fig. I1, Item A). Water flowing or moving under basement slabs is the prime cause of most settlement. A gradual lowering of the local water table accounts for some as well (Fig. I1, Item B)
The recent installation of a nearby storm drain may cause a lowering of the localized water table and may contribute to a slab settlement. Some house slabs actually rest well above the footings, on deep beds of gravel or even sand. A slab settlement on such a house would be much more significant than on a regular house. Slab drops of several feet have been known to occur. When in doubt, “sound” the floor for hollow areas by dropping the end of a solid 4x4 around the settled area and near any cracks. This may help you determine whether additional voids exist under the slab. Jumping up and landing hard on your heels may produce a slab vibration. Occasionally, this indicates that the slab is not resting solidly on the earth beneath it.
WHAT TO: HOW TO:
Slab settlement of one inch or less is usually little to worry about if it has stabilized. “Sound” them to determine that voids are not present beneath. Slabs with voids beneath should be broken up and replaced once the cause of the problem has been identified.
WARNING: Do not rush into breaking up slabs in houses where the slab was originally cast on deep gravel beds, since voids beneath these slabs may be very deep. This is a job for a careful professional. If in doubt, consult with an architect, professional building analyst, professional engineer, or soil scientist.
Mudjacking is a procedure that involves pumping “soil cement” under high pressure beneath slabs to “float” them back up into position. This type of work is very expensive. Consequently, mudjacking should be considered only when the work is large, and breaking up and replacing the slab is not a good choice. Mudjacking is work that is best left to professionals. |
